Gypsum board is a composite material made from two cover sheets or facers (Face/Back) with a gypsum layer (i.e., a gypsum core) sandwiched between the sheets. Physical properties of the facers, facer/gypsum core bond, and strength of the gypsum core, all influence physical properties of the gypsum board.
Conventional methods of preparing gypsum wallboard are well known to those skilled in the art. For example, conventional dry ingredients, wet ingredients, and foam can be mixed together to create a fluid mixture or “slurry.” The dry ingredients can include, but are not limited to, any combination of calcium sulfate hemihydrate (stucco), glass fiber, and accelerator, retarder, and in some cases natural polymer (i.e., starch). The wet ingredients can be made of many components, including but not limited to, a mixture of water, paper pulp, potash, and polymer (hereinafter, collectively referred to as a “pulp paper solution”). The pulp paper solution provides a significant portion of the water that forms the gypsum slurry of the core composition of the wallboard. The foam is pre-generated and continuously fed to the slurry and homogeneously mixed with the slurry.
The slurry is discharged from the mixer through the mixer's outlet chute or “boot”, which spreads the slurry on a moving, continuous top facing material. A moving, continuous bottom facing material is placed on the slurry and the top facing material, so that the slurry is positioned in between the top and bottom facing materials to form the board. The board can then pass through a forming station which forms the wallboard to the desired thickness and width. The board then travels along a belt line for several minutes, during which time the rehydration reaction occurs and the board stiffens (i.e., the stiffening phase). The fluidity of the mix, together with the vibration of the table, will spread the slurry on the top facing material across the board width before the foaming plate. In some cases, the vibration of the table can cause some of the foam air to leave the slurry before it reaches the forming plate.
A conventional gypsum core contains about 60% to 80% air by volume, which depends in part on the components used to form the gypsum core and the amount and structure of foam formed during manufacture of the gypsum core. The gypsum core is formed from a slurry, which is foamed (e.g. air is entrained as the slurry is made by the introduction of foam to the slurry to form air bubbles). As the gypsum core stiffens, the air bubbles are retained in the gypsum core to yield a gypsum core with a plurality of air voids. The size and distribution of the air voids in the gypsum core affects gypsum board strength (e.g. nail pull) and the bonds between the facer material and the gypsum core. The bubbles/air voids can vary in size, shape, and distribution within the gypsum core. The remaining gypsum core between the bubbles/air voids comprises gypsum crystals that form a solid matrix between the bubbles. Typically, the wider the solid matrix between the bubbles/air voids, the stronger the gypsum core. The solid matrix is made from gypsum crystals and to a less extent, starch. In creating the board, it will be appreciated that there is a delicate balance between decreasing weight of the gypsum board while maintaining strength of the gypsum board.
The fluidity of the stucco slurry coming from the mixer can strongly affect the quality of gypsum board. The stiffening and setting time of the slurry should be properly adjusted to keep the slurry fluid enough to spread over the facing material and hard enough to cut at the knife. Stiffening is the change in mix fluidity caused by the hydration reaction. The fluidity of the slurry can be controlled by the amount of dispersant and water added to the slurry and to some extent by controlling the set time. The fluidity of the slurry can be increased by increasing the water to stucco ratio and/or amount of dispersant added to the slurry and vice versa, the fluidity of the slurry can be decreased by decreasing the water to stucco ratio and amount of dispersant used.
While the use of very fluid slurries can be advantageous in certain situations (i.e., with high speed production lines), it can cause some accumulation of bubbles at the top of the gypsum core below the back facing material. Such accumulation can adversely affect the paper-to-core bond and result in high soap usage to maintain the volume of the slurry (the volume of the slurry is known as “head”). Also a very fluid mix can result in air entrainment known in the industry as “core voids”. In contrast to the bubbles that are imparted on the slurry by the foam, core voids are large air pockets (several millimeters in size) that form in the slurry when the slurry has too high of a fluidity. As the slurry is deposited on and spreads over the facing material, the slurry can capture ambient air to form such core voids. Core voids weaken the resulting board and can lead to defective board being produced. These drawbacks can be even more pronounced when unstable foam is used to create larger and more discrete bubbles in the core.
In order to overcome the drawbacks of a slurry with high fluidity, the cohesiveness of the slurry can be increased with the addition of thickening agents. For example, pre-gelled starch can thicken the slurry and increase the slurry cohesiveness. Thickening is increasing mix cohesiveness by adding a thickening material. However, use of pre-gelled starch in this manner results in defoaming which requires manufactures to increase the amount of foaming agent used. Other thickening agents have a slow thickening action and do not work quickly enough to be used in this process.
As such, there remains an opportunity to provide improved slurries, methods of making such slurries, and methods of using such slurries to manufacture improved gypsum boards. Such slurries can be improved by identifying and using a thickener in a manner that improves the cohesiveness of the board and allows proper coalescing to form larger and more discrete air voids in the gypsum layer. As used throughout this disclosure, the terms “air bubble” or “bubble” is used to refer to the bubbles imparted on the slurry by the foam and “air void” or “void” are the terms used to refer to the resulting voids that form from such bubbles in the gypsum core of the finished board. Such use of the terms of “air void” or “void” shall not encompass the defects known as core voids. As used in this disclosure, the terms “air bubble/bubble” and “air void/void” encompasses a bubble, a cavity, pocket, or a void. Further, it will be appreciated that the terms bubble and void can be used interchangeably when discussing the characteristics and size of the bubbles/voids. The use of such thickener should not adversely affect the setting time of the slurry, the paper-to-core bond (wet and dry), or the head of the slurry by acting as a defoaming agent.